Seismic exploration involves surveying subterranean geological media for hydrocarbon deposits. A survey typically involves deploying seismic sources and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological medium creating pressure changes and vibrations. Variations in physical properties of the geological medium give rise to changes in measurable properties of the seismic waves, including their direction of propagation, signal strength, phase, and amplitude, among other properties.
Portions of the seismic waves reach the seismic sensors. Some seismic sensors are sensitive to pressure changes (e.g., hydrophones), others to particle motion (e.g., geophones), and industrial surveys may deploy one type of sensor or both. In response to the detected seismic waves, the sensors generate corresponding electrical signals, known as traces, and record them in storage media as seismic data. Seismic data will include a plurality of “shots” (individual instances of the seismic source being activated), each of which are associated with a plurality of traces recorded at the plurality of sensors. The recorded waveforms (peaks and troughs, often referred to as seismic wavelets) are a quantitative characterization of the geologic boundaries, or subsurface reflectors. Seismic reflection occurs at every location where there is a change in rock or fluid properties. In addition to seismic data recorded in the field, it is also possible to generate synthetic seismic data with a computer that models the seismic sources and computes the propagation of the seismic energy, including reflections, and the seismic data that would be recorded at synthetic seismic sensors.
Seismic data is processed to create digital seismic images that can be interpreted to identify subsurface geologic features including hydrocarbon deposits. Continuous, coherent reflectors seen in the seismic image can be described as complex 3D surfaces with a trackable dip. 3-D digital seismic images may contain a nearly infinite number of these highly complex dipping surfaces.
The seismic wavelets' amplitude and phase respond directly to variations in rock and fluid properties, and depths at which these changes in properties occur are physical boundaries which may be computed from seismic data when they are properly mapped. It is critical that these data be mapped at the highest resolution possible in order to achieve an accurate subsurface description.
Time-lapse (also called 4D) seismic monitoring involves at least two seismic surveys. The first survey is referred to as a baseline survey and usually provides a seismic image of the subsurface prior to production of hydrocarbons contained therein. Subsequent surveys are called monitor surveys. A monitor seismic dataset is representative of the same subsurface volume as the baseline seismic dataset and, in general, undergoes the same processing as the baseline seismic dataset to produce a monitor seismic image. The monitor seismic survey is usually performed months or years after the baseline survey, after hydrocarbons have been produced from the subsurface reservoir, and often after additional recovery techniques such as water flooding or steam injection. Differences between the baseline seismic image and the monitor seismic image indicate subsurface rock and fluid property changes within the reservoir.
The ability to define, at high granularity, the location of rock and fluid property changes in the subsurface is crucial to our ability to make the most appropriate choices for purchasing materials, operating safely, and successfully completing projects. Project cost is dependent upon accurate prediction of the position of physical boundaries within the Earth. Decisions include, but are not limited to, budgetary planning, obtaining mineral and lease rights, signing well commitments, permitting rig locations, designing well paths and drilling strategy, preventing subsurface integrity issues by planning proper casing and cementation strategies, and selecting and purchasing appropriate completion and production equipment.
There exists a need for improved quantification of subsurface changes detectable with time-lapse seismic images that will facilitate enhanced production of potential hydrocarbon reservoirs.